The Worldwide Interoperability for Microwave Access Forum (WiMAX) has developed a specification that describes a radio interface for wireless data communications. This specification is known as the Institute of Electrical and Electronic Engineers (IEEE) 802.16e-2005 standard, and is incorporated herein by reference. WiMAX is intended to provide higher capacity, allow greater communications distances and provide mobility (access across different access points).
Users (and their communications devices known as “subscriber stations”) gain wireless connectivity in an access service network (ASN) via an access point (AP). WiMAX access points (also known as base stations) are similar to cellular access points, with each base station (BTS) generally including a tower with antenna(s) and base station transceiver(s). Once connected, users have the ability to roam from one base station to another. Within the network, each BTS is connected (via wireless or wireline) to a controller node identified as a “gateway” (GW). Each gateway is generally responsible for controlling and communicating with a number of BTSs and is connected to a global network.
Deployment and functioning of WiMAX-based networks faces many challenges that are similar to challenges faced in cellular/PCS networks, however, some of these challenges are more pronounced for WIMAX. Because WiMax operates in high frequency bands (2.5 GHz to 11 GHz), the shadowing effect is more severe. At frequencies around and above 3.5 GHz, line of sight (LOS) is generally required to achieve high data rates. While urban areas are places where high data rates would be beneficial, these urban areas also exacerbate the LOS problem (e.g., buildings, obstacles, etc). Some locations will have no LOS, while other locations will have acceptable LOS around the cell center (BTS location), with poor LOS in areas further from the cell center.
WiMAX is designed for high data rates. Typically, high data rates can only be achieved with high signal-to-noise ratios (SNRs). Because LOS is not possible in most locations, many subscriber stations are severely impacted in locations resulting in no. LOS with low SNR. Often a subscriber station behind an obstacle may acquire the network (i.e., the control channel can be detected), but data throughput rates are low. A high number of users will be in disadvantaged locations that will not support high data rates between the subscriber station and BTS. Therefore, combating the shadow/LOS problem is a major issue in the deployment and operation of WiMax at higher frequencies in urban and dense urban areas.
The shadowing effect, to a lesser degree, is also present in cellular/PCS networks. Several solutions have been proposed to combat this problem in cellular/PCS networks. These solutions include utilizing a higher number of BTSs to cover the same area, over-the-air repeaters and antenna diversity or MIMO (for OFSM/OFDMA systems). Increasing the number of BTSs is costly (e.g., hardware, real estate, antenna towers, backhaul, operating cost). Over-the-air repeaters used in cellular systems are “amplify-and-forward” repeaters that use analog power amplifiers with at least a 10 dB noise figure. Use of such repeaters not only injects significant and unacceptable noise into the sector, these repeaters are costly (e.g., hardware, real estate, operating cost, etc.). Although antenna diversity and MIMO are low cost solutions, these are not effective (e.g., only increase signal by +3 dB, where SNR loss due to shadowing is on the order of −6 dB or greater).
Accordingly, there is needed an effective (and low cost) solution that provides high data rates to subscriber stations in disadvantaged locations.